Method and equipment in a paper/board machine for regulation of the difference of draw

ABSTRACT

The invention concerns an equipment in a paper/board machine for regulation of the difference of draw. In the solution in accordance with the invention, the difference of draw in the web is regulated on the basis of the measured tension of the web. Preferably, the regulation of the difference of draw takes place by regulating the drives at the inlet and/or outlet of a web run not supported by a wire.

FIELD OF THE INVENTION

The invention concerns a method and an equipment in a paper/boardmachine for regulation of the difference of draw.

BACKGROUND OF THE INVENTION

From the applicant's earlier FI Patent No. 80,522, a solution is knownin which the pressure between a curved measurement rib and a web and, onits basis, the tension of the web are measured. The pressure between thecurved rib and the web is passed to a pressure measurement detector, inwhich connection, by means of a pressure-difference detector operatingas the measurement detector, the pressure between the measurement riband the web is measured from a so-called air cushion.

From the applicant's earlier Patent Application FI-930366 an arrangementof a measurement device is known in which the cross-direction variationin the web tension can be changed to the desired level. The web is madeto run over a so-called airborne rib, which consists of a curved face,in which connection the air pressure formed between the airborne rib andthe web is measured, and, on the basis of said measurement values,positions signals are passed to an actuator, in which connection thecross-direction variation in the web tension can be changed to thedesired level.

However, the measurement of tension is not used for regulation of thedifference of draw in the web in any of the prior-art solutionsdescribed above.

Determination of the difference of draw has taken place mainlyempirically. In such a case, for example, in connection with a freedraw, visual observation has been used for determination of thedifference of draw. Separation of the web in connection with a free gap,for example, from the centre roll in a press section has been observed.

OBJECTS AND SUMMARY OF THE INVENTION

In the present patent application, it is suggested that the differenceof draw be determined and regulated based on measurement of the tensionof the web.

In an embodiment of the invention, the measurement of tension takesplace by means of a so-called weighing roll. At the ends of the weighingroll, the loading of the roll is measured by means of detectors, and,when the load becomes higher as the web loads the roll with a certaintension, the web tension can be computed directly from the increase inloading measured by means of the detectors. In an embodiment of theinvention, measurement of the web tension is based on measurement of thepressure in an air space between the web and a backup face.

Thus, in the commonest embodiment of the invention, the difference ofdraw is regulated based on the result of measurement of the web tension.The difference of draw is preferably regulated in a paper/board machineby regulating the motors of the drives at the inlet side and outlet sideof a free gap. Advantageously, the measurement is carried out betweenthe drives at the inlet side and outlet side of a so-called free gap,i.e. a gap not supported by a wire. For measurement of the web Wtension, it is possible to use a weighing roll, in the way describedabove, in which connection the load applied by the web to the roll isobserved by means of detector devices of the weighing roll, and on thisbasis the tension of the web at the roll is determined. In the waydescribed above, measurement of the web tension can be based, forexample, on measurement of a pressure which is measured between a backupface in the free gap, for example a roll, and the web. Thus, it has beenrealized to use measurement of pressure for fitting together of the webtension profile and the difference of draw. From the air space betweenthe web W and the measurement face, the air pressure is measured, and bymeans of said pressure data the draw difference of the web is regulatedso that the desired tension is obtained for the web. The measurementface can be a separate face of a measurement rib or the face of aseparate roll. In such a case, the pressure is passed from between theroll face and the web into a pressure space placed in the interior ofthe roll and further to a measurement detector, which may be placedseparately or in connection with said pressure space in the roll. Theroll may be revolving or non-revolving. The pressure can be measuredfrom each hole separately (tension profile in the cross direction of themachine), or several holes can communicate with one measurementdetector.

When a non-revolving roll is used, the construction of the measurementdevice may consist of a roll in which the measurement chamber is fittedin the interior of the roll and holes have been fitted to be opened intothe roll interior over a certain sector across the entire width of thesector. In such a case, the measurement pressure can be obtained from awider area, and a better average value can be obtained from themeasurement. When a revolving roll is used, it is enough thatperforations have been made through the roll mantle into the measurementchamber in one row in the longitudinal direction of the roll.

When the web has a speed V₁ in the first nip and a speed V₂ in thesecond nip, the relative difference in speed that is obtained is:$ɛ = \frac{V_{2} - V_{1}}{V_{1}}$

When the relative difference in speed ε becomes higher, the tensionproduced in the web, i.e. the web tension T, also becomes higher. With acertain value of ε the web is broken, and, on the other hand, when ε istoo low, i.e. the difference in speed is not sufficient, flutteringarises in the web.

The web tension T is formed by the sum T₁+T₂, wherein the tension T₁ iscomputed from the formula: $P = \frac{T_{1}}{R}$

wherein P is the pressure applied to the web, and R is the curved faceover which the web is passed, for example a roll used for guiding theweb. T₂ is composed of a mass and speed term, in which M is the bas isweight of the web and V is the speed of the web. Thus, for the overallexpression, the value T=P·R+MV² is obtained.

When the speed of the web and the pressure applied to the web are known,from the above formula it is possible to compute the overall tension ofthe web, and, based on said computed value, if necessary, it is furtherpossible to adjust the difference in speed, i.e. it is possible to actupon the values V₁ and V₂ in the vicinity of the measurement point.

In a preferred embodiment of the invention, the tension of the web isestimated by passing the web over a curved measurement face, for examplea roll, and by measuring the pressure P present between the web and themeasurement face. Based on the measured pressure, the tension of the webis detected at the measurement point and preferably, in a paper/boardmachine, at a free and unsupported draw of the web, and based on themeasurement data thus obtained, the difference of draw between the inletside and the outlet side of the free gap is regulated, for example, byregulating the drives of rolls at the inlet side and at the outlet side.

In the present patent application, the invention is not supposed to beconfined exclusively to such regulation of a difference of draw and,thereby, to such regulation of the web tension in which the measurementdevice is placed exclusively in connection with a roll. As was describedabove, the web may also be made to run over a curved rib face orequivalent, which rib face has a certain curve radius and from which ribface flow openings are opened through the rib across the width of theweb, said openings passing into a pressure monitoring space placed atthe opposite side of the rib.

In the solution in accordance with the invention, the web tension isregulated, on the basis of the pressure measured at a free draw(unsupported draw) of the web between a measurement face and the web, byregulating the difference in speed between the drive motors placed atboth sides of the free gap.

In accordance with the invention, the difference of draw is regulatedpreferably at the drives between the third press and the fourth press,between which presses the web has a free draw (an unsupported web draw),and favourably said difference of draw can also be regulated between thefourth press and the first drive motor of the dryer section, betweenwhich drives there is also a free draw of the web.

Thus, based on the difference in pressure between the web and ameasurement face, the web tension is measured, and based on said data ontension, the difference of draw is regulated preferably between a drivethat precedes a free draw of the web and a drive following after saidfree draw by regulating the speeds of rotation of the motors of saiddrives.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in the following with reference to somepreferred embodiments of the invention illustrated in the figures in theaccompanying drawings, the invention being, yet, not supposed to beconfined to said embodiments alone.

FIG. 1A illustrates a principle of measurement, wherein the web is madeto run along the measurement face, preferably the face of a measurementrib, and in the solution of equipment the pressure is measured from theair space placed between the web and the face of a measurement rib.

FIG. 1B illustrates a second solution of the mode of measurement,wherein the pressure is measured from between the web and the face of aroll while the roll comprises perforations through which the pressure ispassed to the detectors.

FIG. 1C shows such a roll for use in measurement as comprisesperforations provided in the roll face on a certain sector, in whichconnection the pressure can be measured from the area of a certainsector.

FIG. 1D illustrates a so-called weighing roll.

FIG. 2 illustrates the principle of carrying out the regulation of thedifference of draw in accordance with the invention.

FIG. 3A illustrates an application of the principle of regulation asshown in FIG. 2, in accordance with the invention, in the press sectionof a paper machine.

FIG. 3B shows a second embodiment of the regulation in accordance withthe invention, wherein the difference of draw is regulated between thedryer section and the last press, i.e. a so-called separate press, inthe press section.

DETAILED DESCRIPTION OF THE INVENTION

As is shown in FIG. 1A, the web W is transferred over the measurementface 100. In the embodiment shown in the figure, the measurement face100 is the outer face of a measurement rib 110. From the measurementface 100, a duct 120 passes to the detector 170, in which connection thepressure P is measured from the air space E between the web W and themeasurement face 100. Thus, based on said pressure data, the tension ofthe paper web W is obtained indirectly. Based on said tension data, theweb tension is regulated further to the desired level by regulated thedifference of draw between the drives provided at both sides, i.e. atthe inlet side and at the outlet side, of the so-called free draw of theweb W.

FIG. 1B illustrates a second embodiment of the mode of measurement inaccordance with the invention, wherein the web is made to run along theface 150 of the roll 140, and the pressure is measured from the airspace E between the web W and the face 150. As is shown in the figure,the roll 140 includes perforations 160 a ₁, 160 a ₂ . . . on its face,through which perforations the pressure is passed to the detector 170 inthe measurement chamber C. The roll can be revolving or non-revolving.The holes 160 a ₁, 160 a ₂ . . . are opened into the measurement chamberC in the interior of the roll, in which chamber the detector/detectorsthat 170 measures/measure the pressure p is/are placed.

In the solution in accordance with the invention, each perforationextending across the width of the roll or a certain group of holesacross the distance of the length concerned can comprise a separatedetector, in which case it is also possible to determine the tensionprofile of the web across the width of the web.

FIG. 1C shows an embodiment corresponding to FIG. 1B, but in theembodiment of FIG. 1C the measurement data are passed within a certainsector S to the measurement detector, in which connection the pressuredata obtained correspond to a certain average pressure value over awider sector-shaped measurement area. The roll 140 shown in the figurecan also be non-revolving. The holes 160 a ₁, 160 a ₂ . . . are openedover a wider sector area into the measurement chamber C in the interiorof the roll 140, in which chamber the detector 170 for measurement ofpressure is placed.

FIG. 1D shows a so-called weighing roll 200, by whose means it ispossible to measure the tension of the web W. Loading detectors 200 a ₁and 200 a ₂, have been fitted at the ends of the roll 200. By means ofthe detectors 200 a ₁ and 200 a ₂, the increase in load produced by theweb W is measured, and thereby it is further possible tocompute/determine the tension of the web W at the roll 200.

FIG. 2 illustrates the main principle of the method in accordance withthe invention. Between the press nips N₁₀ and N₂₀ of the presses P_(I)and P_(II), there is a free draw F, in which connection the pressure pis measured from the space E between the measurement face 100 of the rib110 of the web, and by means of said measured pressured the differenceof draw V₁-V₂ between the driven presses P_(I) and P_(II). V₁ is thespeed of the web W at the press P_(II), and V₂ is the speed of the web Wat the press P_(I). Thus, as shown in the figure, the speed(s) ofrotation of the drive motor M₁₀ of the rolls T₁ and T₂ that form the nipN₁₀ and/or the drive motor M₂₀ of the rolls T₃ and T₄ that form the nipN₂₀ is/are regulated. It is the main principle that the difference inspeed V₂-V₁ of the web between the nips N₁₀ and N₂₀ is adjusted byregulating a drive motor. In order that a tension could be formed in theweb W over the free gap F, the speed V₁ of the web w in the nip N₂₀ andin the press P_(II) is higher than the speed V₂ of the web W in the nipN₁₀ and in the press P_(I).

FIG. 3A illustrates the method in accordance with the invention asapplied to a press section P. The web W is passed between the felts H₁and H₂ on arrival in the press section P. The felt H₁ is passed over thefelt guide rolls 10 a ₁, 10 a ₂ . . . 10 a _(n), and similarly the feltH₂ is passed over the felt guide rolls 11 a ₁, 11 a ₂ . . . 11 a _(n).The web is passed into the first press 12 in the press P, which press 12comprises the press rolls 12 a ₁, 12 a ₂. From the nip N₁ the web istransferred further into the second press 120, into its nip N₂ formedbetween the centre roll 13 and the press roll 12 a ₁, and further, alongthe centre roll 13, into the third press 14 in the press section P, intothe nip N₃ formed between the press roll 14 a ₁ and the centre roll 13.After this the web W is transferred from the nip N₃ of the third press14 through a free draw F₁ onto the transfer roll 15. The transfer roll15 comprises a measurement face 15′, in which connection, in thesolution of equipment, the pressure is measured from between the web Wand the measurement face 15′, and by making use of said pressure data,the tension of the web W is determined, and based on said determined webW tension, the difference of draw V₁ -V₂ in the web W and, thus, thetension of the web W, are regulated to the desired level.Advantageously, the regulation of the difference of draw V₁-V₂ in theweb is carried out between the drive at the inlet side and the drive atthe outlet side of the so-called free draw, i.e. unsupported draw F₁, ofthe web. The web W is passed further from the roll 15 into connectionwith the transfer felt H₄ and along with said felt into the fourth press16, into the nip N₄ formed between the press rolls 16 a ₁ and 16 a ₂.The felt H₄ has been passed over the felt guide rolls 17 a ₁, 17 a ₂ . .. 17 a _(n). Similarly, the felt H₃ has been passed over the felt guiderolls 170 a ₁, 170 a ₂ . . . In the regulation of the difference of drawV₁-V₂, in the embodiment shown in FIG. 3A, V₁ is the speed of the paperor board web W at the outlet side of the free gap F₁ at its first drive,in the embodiment of FIG. 3A at the driving nip N₄ of the fourth press16. V₂ is the speed of the paper or board web at the inlet side of thefree gap F₁ at the first drive related to its inlet side, in theembodiment of FIG. 3A at the driving nip N₃ of the third press 14.

From the fourth press 16, the web W is passed further through the freedraw F₂ onto the transfer roll 18 and further into the dryer section.The air pressure p is measured from between the face 15′ of the transferroll 15 and the web W, and, based on the measurement data, thus, thetension of the web W is determined indirectly, and on its basis the webW tension is regulated further by regulating the difference of draw.

The web is passed further from said second transfer roll 18 into thedryer section R into its first drying group R_(I) onto the first dryingcylinder K₁ in said group along with the lower face of the wire H₅ ofthe single-wire draw, for example, supported by a vacuum suction.

As is shown in FIG. 3A, the difference of draw is regulated byregulating the speed of the web W between the roll 16 a ₁ of the fourthpress 16 and the roll 14 a ₁ of the third press 14. In accordance withthe invention, the drive motor M₂ of the fourth press 16, preferably themotor M₂ that drives the roll 16 a ₁, is regulated. Before this,information concerning the operation has been received from the drivemotor M₁ of the third press 14, preferably from the drive motor M₁ ofthe roll 14 a ₁, concerning its speed of rotation. The regulation can becarried out by regulating the motor M₁ or M₂ or both of said motors.

FIG. 3B illustrates a second embodiment of the invention, in which thedrive motor M₃ of the first drive in the dryer section R, i.e. of thedrive of the drying cylinder K₁, and the drive motor M₂ of the pressroll 16 a ₁ of the fourth press 16 are regulated. For the purpose ofregulation, the data concerning the speed of rotation of the drive motorM₂ of the press roll of the fourth press 16 are passed into the centralunit 1000, and, based on the data on the pressure in the air space Ebetween the web W and the measurement face 100 of the measurement rib110, the tension of the web is determined. The data concerning this aretransferred into the central unit 1000, and, within the range betweenthe maximal and minimal values of the tension, an instruction of speedis regulated for the drive motor M₃ of the drying cylinder K₁ of thefirst drying group R₁ in the dryer section R so as to operate the dryingcylinder K₁ at a certain speed of rotation. Regulation of the differencein speed, i.e. of the difference of draw, V₁-V₂ can be carried out byregulating the speed of rotation of the motor M₃ or M₂ or by regulatingboth of the motors M₂ and M₃. In stead of a measurement rib 110 it isalso possible to employ a roll 18, as is illustrated in FIGS. 3A, 1B and1C, or the tension can be measured in said position by means of aweighing roll 200. When the difference of draw V₁-V₂ is being regulated,in the embodiment of FIG. 3B, V₁ is the speed of the paper or board webW at the outlet side of the free gap F₂ at the drive first connectedwith the free gap, in the embodiment of the figure, at the drive of thedrying cylinder K₁. V₂ is the speed of the paper or board web W at theinlet side of the free gap F₂ at the drive most closely connected withthe inlet side of the free gap F₂, in the embodiment of FIG. 3B, at thenip N₄ of the fourth press 16.

We claim:
 1. An arrangement in a paper/board machine for regulating thedifference of draw of a web, said arrangement comprising: means fordetermining the tension of the web; and means for adjusting thedifference of draw based on said determined tension of the web; whereinsaid means for adjusting the difference the draw comprises first andsecond motors of first and second nip rolls arranged respectively at aninlet side and outlet side of a free gap of the web; and wherein saidmeans for determining the tension of the web is arranged in the free gapof the web between said first and second motors.
 2. The arrangementaccording to claim 1, wherein said means for determining the tension ofthe web is an air pressure detector structured and arranged to measurean air pressure between the web and a roll, whereby based upon airpressure a tension of the web is calculated.
 3. The arrangementaccording to claim 1, wherein said means for determining the tension ofthe web is a plurality of load detectors arranged on a roll formeasuring a load applied to the roll by said web, whereby based uponsaid measured load a tension of the web is calculated.
 4. Thearrangement according to claim 1, wherein said web is passed into apress section into and through a first press and into a second pressformed between a center roll of the press section and a press roll ofsaid first press, said web being passed into a third press formedbetween said center roll and the press roll, said web passed through afree draw and over a transfer roll in to a fourth press and from thefourth press through a second free draw and over a second transfer rollinto a dryer section, wherein said means for determining the tension ofthe web is arranged to measure the tension of the web between said thirdand fourth press and said means for means for adjusting the differenceof draw is arranged to regulate the difference of draw between saidthird and fourth press.
 5. The arrangement according to claim 4, whereinsaid means for determining the tension of the web is an air pressuredetector.
 6. The arrangement according to claim 5, wherein based uponsaid measured tension of the web a drive motor of the fourth press and adrive motor of the third press is regulated.
 7. The arrangementaccording to claim 1, wherein said web is passed into a press section(P) into and through a first press and into a second press formedbetween a center roll of the press section and a press roll of saidfirst press, said web being passed into a third press formed betweensaid center roll and the press roll, said web passed through a free drawand over a transfer roll in to a fourth press and from the fourth pressthrough a second free draw and over a transfer roll into a dryer sectionand over a first drying cylinder, wherein said means for determining thetension of the web is arranged to measure the tension of the web betweensaid fourth press and said first drying cylinder, and said means foradjusting the difference of draw is arranged to regulate the differenceof draw between said fourth press and said first drying cylinder.